1. Statement of the Technical Field
This invention generally relates to orbital shaker apparatus and, more specifically, to an apparatus for reducing the instability generally caused by static imbalance between a counterweight and the load of flasks or other vessels on the platform, and an apparatus for varying the orbit diameter of the shaker.
2. Description of the Related Art
An orbital shaker apparatus is a mixing or stirring device used especially in scientific applications or mixing or stirring containers, such as beakers and flasks holding various liquids on a platform. Specifically, an orbital shaker translates a platform in a manner such that all points on the upper surface, in the X-Y plane, of the platform move in a circular path having a common radius. Generally, beakers, flasks, and other vessels are attached to the upper surface of the platform such that the liquid contained therein is swirled around the interior side walls of the vessel to increase mixing and increase interaction or exchange between the liquid and local gaseous environment. Conventionally, the apparatus which drives the platform in an orbital translation includes one or more vertical shafts driven by a motor with an offset or crank on the upper end of an uppermost shaft such that the axis of the upper shaft moves in a circle with a radius determined by the offset in the shaft, i.e., by the “crank throw”. The upper shaft or shafts are connected to the underside of the platform via a bearing to disconnect the rotational movement between the upper shaft or shafts and the platform.
In operation, the mass of the shaft above the offset or crank throw, the platform with its mounting hardware and the load consisting of the filled flasks or vessels, and the clips or fasteners which hold the vessels to the platform all translate at the rotational velocity of the driven shaft in a circle with a radius equal to the crank throw. The mass of the liquid within the vessels translates at the shaft rotational velocity in a circle with a radius equal to the crank throw plus the distance from the center of the vessel to the center of mass of the liquid contained in the vessel.
The forces resulting from the total orbitally-rotating mass can often cause motion of the base of the shaker which can superimpose additional motion components into the liquid in the vessels and lead to undesirable turbulence or splashing. These forces can also cause the base unit to move or “walk” along its support surface.
In order to reduce this motion, the mass of the non-rotating supporting structure must be increased to resist the forces generated by the rotating mass. This leads to the undesirable effect of increasing the overall weight of the shaker simply to address for stabilization. Alternatively, counterweights have been employed to oppose or compensate for the forces generated from the orbitally-rotating mass.
For example, U.S. Pat. No. 3,430,926 to Freedman, et al., entitled “Counterweight System for Shaker Apparatus,” describes the use of multiple fixed counterweights situated about a shaft which counteract the imbalance forces generated by a rotating platform.
U.S. Pat. No. 5,558,437 to Rode, entitled “Dynamically Balanced Orbital Shaker,” addresses the issue of static and dynamic imbalance by positioning various fixed masses in the plane of the crank arm such that their masses and placement exactly cancel out the effects of the rotating platform's mass contribution.
Similarly, European Patent Application No. EP1854533 to Hawrylenko, entitled “Shaker,” describes a crank arrangement where two balancing masses can be adjusted radially and vertically to compensate for a given loading condition.
These arrangements all undesirably require selecting specific masses and locations, vertically as well as radially, which vary depending upon the platform load conditions. In addition, in order to correct for large mass imbalances statically and dynamically, these devices require considerable space to place the correcting weights in the appropriate locations relative to the platform load, and also increase the overall product weight.
U.S. Pat. No. 6,106,143 to Nickel, et al., entitled “Vibrating Device for Vibrating Liquid Provided in Vessels,” provides a means to adjust a static counterweight to compensate for a range of platform loads by advancing or retracting a mass radially along an axis. The distance between the center of mass of the counterweight and the axis of rotation increases or decreases, and thus generates an increased or decreased amount of balance compensation. This is a practical solution for modest platform loads but is not feasible for providing a large dynamic compensation range. For example, if a large counterweight mass is selected, it may not be positioned close enough to the axis of rotation to achieve a minimal balance compensation. If a small counterweight is selected, it is difficult to position it far enough from the axis of rotation to balance a large platform load without using considerable additional space. Also, this device does not provide any feedback to the user that the onset of detrimental instability is imminent, which would require a compensating adjustment.
U.S. Patent Application Publication Serial No. US2008/0056059 to Manera, et al. describes the use of a vibration sensor to detect an unbalanced loading condition and reduce the shaking speed to a stable magnitude, but it does not provide a means for the counterweight of the orbital shaker to be adjusted, or a process which can be applied, in order to achieve the desired speed.
There are rotating equipment in other technical fields that use balancing heads to correct for rotor imbalances using two arms with weights. See, for example, Mechanical Vibrations, J. P. Den Hartog 1934, pp. 236-237 ISBN 0-486-64785-4. However, orbital shakers tend to differ because the platform load includes not only a static mass component, but also a dynamic component, namely the fluid in the flasks or other containers. This fluid generates a variable imbalance depending upon the geometry of the container, amount of fluid in the container, the orbit diameter of the shaker, and the speed of the shaker which could result in a different amount of resultant balance compensation depending upon the operating conditions. Furthermore, automatic balancing techniques, whereby balancing masses migrate to the correct positions to minimize vibrations, are not generally applicable to orbital shakers because orbital shakers operate much slower than the critical speeds required to enable these techniques.
The eccentric throw for an orbital shaker is typically fixed by precisely machining a single component. The offset between the two eccentric journals defines the orbit radius. This radius is not adjustable. Adjusting the eccentric throw by separating the two journals into independent bearing housings whose centers of rotation can be fixed at different eccentric offsets relative to each other is a method which is known in prior art, such as, for example, the Kuehner shaker. What has not been achieved is a means of manually or automatically adjusting the eccentric throw within a continuously variable range. Furthermore, a change in the eccentric throw for a given platform load results in a change in the amount of counterweight needed to compensate for it. Thus, it is desired to combine the ability to adjust the eccentric throw with the ability to adjust the compensating counterweight simultaneously.
It is also desirable for an orbital shaker device to provide feedback to the user, or, in the case of a shaker with automatic adjustment, to provide feedback to its controller that the onset of detrimental instability which would require a compensating adjustment is imminent.
It is also desirable to provide an orbital shaker capable of balancing a large platform using counterweights of intermediate size without requiring a device of unreasonable size or weight.